You are in a system as a couple, and while you ultimately both need to be working on the relationship, one of you has to begin, even with baby steps. When you feel disconnected from your partner, everything feels worse.
Caution: Do not grind the green shoots with your heel. What does that mean? When one of you tries a new behavior, and your trust is low, it's tempting to crush the effort verbally, or by turning away. That might be your experiment of new behavior -- not to stamp out the effort of your partner.
So what might you try? I'll give some ideas here, but this is a only a beginning. You know your partner best.
- Think back to when you were happy and in love. You did small things for each other; what were they? Try those.
- Slow down the conversation; ask questions to show your interest in your partner.
- Listen while s/he's talking (vs. preparing your reply).
- Take three deeps breaths before responding (vs. reacting).
- Say, "That didn't go very well; let's try again."
- Be explicit vs. vague about plans or expectations (you won't always get what you want, and that's ok, too).
- If things get heated, take a break for 20-30 minutes and come back to talk for a specific amount of time (e.g., 20 minutes)
- Spend 15 minutes asking and listening to your partner about his/her day (listen deeply). Make sure both of you talk.
- When a nasty comment pops up, don't say it. Instead notice how you feel (hurt, annoyed, etc.) and say that: "I feel hurt."
- Go for a walk together (or get exercise on your own). Notice what you see, hear, and smell.
- Give each other the benefit of the doubt; your partner's intentions are probably better than you think.
- Remember that no matter how long you've known each other, actions and behaviors have different meanings to each of you. Do not ASSUME you know the meaning your partner holds. Ask, "What does that mean to you?"